When their money is at stake, people often ask what the downside is. How much can I lose? Then they weigh this against the upside. How much can I make?
Strangely, when money isn’t at stake, people don’t really seem to consider the second question. A lot of people don’t hesitate to be the bearer of bad news, especially when it comes to personal matters. Ally makes a snide comment about Bob, and Cathy immediately informs Bob. What’s the downside, right? If Cathy doesn’t tell say anything, maybe Bob will find out and then be mad at Ally and Cathy.
But on the other hand. What’s the upside? Now you’ve guaranteed that Bob’s going to feel shitty, or angry, or hurt, or all of the above.
Most of the time, there is no upside. (And, when you really look at it, there’s hardly ever a downside in these situations: Bob’s never going to be mad at Cathy for something Ally said. And if he is, fuck Bob. He’s an idiot.)
So ask yourself: What’s the upside? If the answer isn’t obvious (and doesn’t outweigh the downside), then keep it to yourself.